Thursday, 4 July 2013

When exposed to humour, women's brains exhibit more activity than men's in reward-related regions. Some experts say this is consistent with an idea derived from evolutionary theory that women are predisposed to be humour appreciators whereas men are humour producers. According to this view, women use a man's comedic skills as a way to appraise his genetic fitness.

An obvious objection here is with the word "predisposed". Who's to say whether these gender differences are innate or if they're a result of cultural influences? A new study has started to answer this question by scanning the brains of girls and boys as they viewed funny videos - the first time that gender-related brain differences in response to humour have been examined in children.

Pascal Vrticka and his colleagues showed the funny clips, including people falling over and animals performing tricks, to 22 healthy children - 13 girls, 9 boys - aged from six to thirteen (data from a further ten children was lost because they moved about too much in the scanner). For comparison, the children also watched "positive" clips, featuring dancers and snowboarders among other things, and neutral clips, which featured nature videos and kids riding bikes. As they watched the clips the children's brains were scanned with fMRI. The children also said how much they enjoyed the clips and how funny they found them.

In response to funny clips (versus positive clips) the girls' brains showed more heightened activity than the boys' in a range of areas including bilateral tempero-occipital cortex, midbrain and amygdala. What does this mean? "This finding indicates that girls more readily engaged in incongruity resolution and experienced stronger mirth, positive feeling state, and/or reward representation during humour appreciation," said the researchers. This shows, they added, that the humour-related gender differences found in adult brains already exist in young children.

In contrast to the situation for funny clips, the boys' brains showed a stronger response than girls' brains to the positive clips, including in ventromedial prefrontal cortex and amygdala. The researchers said this shows the boys expected reward, not just in the funny clips but in the positive unfunny clips. Because they anticipated reward, Vrticka's team said the boys' experience of mirth was diminished when a joke actually came. Girls, by contrast, were more surprised by the humour in the funny clips, which therefore brought them more reward.

The same results applied when analysis was restricted to the portion of participants who were opposite-sex siblings raised in the same homes. This strengthens the case that it is at least partly biological differences that lie behind the gender differences reported here (but this is far from conclusive: opposite sex siblings can be exposed to different environmental influences whether raised together or not).

The researchers said their findings support the idea that women have evolved to be humor appreciators: "... the extant neuroimaging data support the notion that mate selection by means of humor processing might be more effective in females than males because the female brain, and particularly the reward circuit, is biologically better prepared to respond accordingly."

There's one very important problem with all this. The girls didn't actually like or find the funny videos any funnier than the boys. Their super efficient joke-sensitive neural reward circuits were firing away, but this wasn't translated into actual amusement. Is this a classic case of researchers treating brain imaging evidence as somehow more true or fundamental than behavioural data?

Here's how they explained the lack of gender differences in the kids' ratings of the videos: "... differential brain activity for funny versus positive clips most likely reflects a sex bias for distinct processing mechanisms, rather than divergent subjective experience." OK, but how do they get from that interpretation to their conclusion that "our data for the first time disclose that sex differences in humour appreciation already exist in young children"?

Yes, this small study suggests girls' and boys' brains respond differently to funny videos. But there were no differences in subjective humour appreciation between the sexes and so, contrary to the researchers' interpretation, the study could be taken as another example of how the brains of men and women (and boys and girls) sometimes take a different route to the same end result.